why some Plc used two CPU?

Hi guys,
I am sorry for this type of question. I observed in my company some Plc used only one CPU and some machine used two CPU. My concept is not clear that's why I would like to why used two CPU.we are using Allen Bradley Rslogix 5561.
Any answer will be highly appreciated.
Kind regards.
Jamal.

from the picture of the rack you are using SERCOS motion
it is probable that the second CPU is needed so as to keep the motion scan times reasonable or you have one axis group that must respond much faster than the other

PS replace the battery in the second processor

Hi Michaelg,
Thanks for your answer. I am very happy to know this information.
Now my 2nd question is 2nd procerror I don't see any battery compartment. But first processor I see there is battery compartment and there is two battery and this is connected
With first processor.so my doubt is still not clear. But I don't pull out the 2nd processor to see the battery.
Any help kindly.
Regards.
Jamal.

Yes we have 8 servo drive kintex.

open the door under the key in the processor

Manual
http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1756-um001_-en-p.pdf
goto page 38

Sorry I couldn't see any battery is there. But still i am reading the manual.

Those empty clips above the serial port are where the battery is mounted. There is a 3-pin connector in there as well, to plug in the battery.

That processor is a 1756-L61 Series A. You can tell because the CompactFlash card port is on the bottom instead of in the front.

The battery for the 1756-L61 Series A is part number 1756-BA1.

That is the same battery assembly part number that was used for the 1756-L1 and 1756-L55 controllers, so it goes back to 1998; they are widely available from A-B distributors worldwide.

The newer 1756-L6x Series B controllers use a blue-jacketed battery with a different plug, part number 1756-BA2. The Series B controllers notably have the CompactFlash port right in the front of the processor, to the right of the serial port.

Ist processor there is battery module and there is two battery, OK fine.but 2nd processor battery place is empty and battery plug in point there is no battery cable.CPU is giving battery fault indicate but m/c is working fine.but my confusion Is not clear.I have to fixed battery or not?
Any suggestions will be helpful.

The cable comes pre-soldered to the battery and has a connector on the end of it which plugs into the CPU PCB. The PLC will continue to run but if there is power loss you might then have problems so you should order and fit a new battery.

As the guys are pointing out, the pins you see on the front of the processor are a socket connection for the lead and plug that come pre-soldered to the battery. The two rounded clips are retainers for the battery. It's possible that someone removed the old battery when it showed a fault so they could order a new one and they never followed up on it and that's why it's missing now?

Here's a sample image of how the 1756-BA1 comes...



It's not essential to have a battery, but very good practice to have a healthy one in case of brownouts or blackouts. You don't want to have to be reloading the program for every power loss or machine power cycle. Another reason why the SD Card is useful.

Why would there be two Logix processors in the same chassis?

Motion, yes. As you can only have one Motion group per Logix 5000 processor, there can often be the need to segregate into two or more groups.

You could also have an Enhanced Redundancy system where a failure on the first processor will failover to the second processor.

Or there could be two or more separate applications, within the same system, that require the same local I/O data (Owner, Input Only, Listen Only, etc.) all on the same backplane.

Or a second processor may just be used in the same chassis as a data collector or concentrator, and pass the data out to a SCADA system, say through a 1756-ENxT module.

For standard applications, there is no limit on the number of processors which may reside in the same Logix 5000 chassis, except for the limit on the power supply's 5v backplane draw.

Regards,
George